1. Field of the Invention
The present invention relates to a developing device for developing an electrostatic latent image formed on an image carrier with a liquid developing agent, which is employed in printers, photocopiers, facsimile devices, or the like.
2. Description of the Related Art
Conventionally, various types of image formation apparatuses have been proposed, wherein a thin film of a liquid developing agent is formed on the surface of a developing roller or a developing belt, serving as a developing agent carrier, following which the thin film of the developing agent is moved to a portion facing a photosensitive member serving as a latent-image carrier so as to be in contact with the photosensitive member, thereby forming a visible image from the electrostatic image formed on the photosensitive member. As a liquid developing agent employed in the aforementioned image formation apparatuses, an arrangement is known wherein toner (solid toner) is dispersed in a liquid solvent.
The aforementioned image formation apparatus may have a configuration wherein a film of a liquid developing agent is introduced between the photosensitive member and the developing agent carrier with a small gap so as to move the toner in the liquid developing agent to the photosensitive member. Accordingly, a visible image is formed from an electrostatic latent image on the photosensitive member, whereby an image (toner image) is formed with high resolution and excellent dot reproductivity.
FIG. 5A is a schematic diagram which shows the movement of the liquid developing agent at the time of developing. As shown in FIG. 5A, an electroconductive developing roller 13 having elasticity is employed as the developing agent carrier, and a thin film of the liquid developing agent is formed on the surface of the developing roller 13 using a blade. Furthermore, members such as spacing rollers or the like are provided at the ends of the developing roller 13 or a photosensitive drum 1 such that a small gap is formed at the developing portion D between the photosensitive drum 1 and the developing roller 13. The toner dispersed in the liquid developing agent forming a thin film on the developing portion D is adjusted so as to be charged with a predetermined magnitude. For example, the toner is charged positively in the example shown in FIG. 5A. The surface of the photosensitive drum 1 is uniformly and positively charged by charging means, following which the charges on the exposed portion (image portion) are removed by exposure means. Subsequently, a DC bias voltage serving as a developing bias voltage is applied with a potential greater than that of the exposed portion so that the developing agent is adhered to the exposed portion, whereby so-called inverse developing is performed.
However, such conventional developing devices employing the liquid developing agent have problems as follows.
In order to reduce the size of the entire developing device employing the liquid developing agent, the amount of waste liquid, as well as the amount of the liquid developing agent for developing, must be reduced. Accordingly, the liquid developing agent is preferably adjusted such that the concentration of the solid toner is increased, resulting in an increase of the viscosity of the liquid developing agent. That is to say, the liquid developing agent with high viscosity is required for such an apparatus. On the other hand, in a case of employing a volatile carrier liquid for the developing agent, there is a problem that air pollution might result from gasses emitted by the volatile carrier liquid. Taking such a problem into consideration, a nonvolatile carrier liquid is preferably employed. Accordingly, the carrier liquid is preferably adjusted to have high viscosity, and specifically, the viscosity thereof is preferably 100 mPa·s or more (see Japanese Patent Laid-Open No. 7-209922, and Japanese Patent Laid-Open No. 2001-356607).
As described above, from the point of reduction of the size of the apparatus, and environmental concerns, a liquid developing agent with high viscosity is preferably employed in the developing device. However, the developing device employing the developing agent with high viscosity has a problem that fogging can occur due to adhesion of the toner contained in the liquid developing agent to non-image portions, as with the portion E shown in FIG. 5B. Furthermore, the developing device employing the developing agent with high viscosity has another problem that a uniform thin film of the toner cannot be formed on the developing roller 13 due to the high viscosity of the carrier liquid, and accordingly, an image with a uniform density cannot be formed on a region where uniform exposure has been made.
As described above, the developing device having the conventional configuration wherein the liquid developing agent with high viscosity is employed from the point of reduction of the size thereof and environmental concerns, has the problem that fogging may occur due to adhesion of the toner onto non-image portions, and the problem that an image with a uniform density cannot be formed on a region where uniform exposure has been made.